The abrupt resignation Monday of Nissan CEO Hiroto Saikawa, after an internal investigation uncovered falsified documents that boosted his compensation, marks a dramatic shift in an ongoing scandal that broke open last November with the arrest of then-Chairman Carlos Ghosn.

Suddenly, Saikawa — who ordered the investigation into his former mentor — finds himself in the spotlight and standing accused of pocketing excess pay, too.

It also compounds the woes facing a company that appears to have done little in the past to monitor or rein in its top executives. And the lack of oversight…